An Extinction Rebellion activist had to be taken off a flight at London City Airport, as part of ongoing protests.
BBC Newsnight’s Nicholas Watt said his flight was about to take off when a protester refused to resume his seat.
Meanwhile, police made several arrests as activists blocked the main entrance to the airport, with some gluing themselves to the floor.
In Westminster, tents and protesters have been cleared from the roads leading to Parliament Square.
However, streets around Parliament and Whitehall remain closed to traffic, apart from cyclists, with a heavy police presence in the area.
It is the fourth day of the Extinction Rebellion climate change protests, which are taking place across central London and other major cities worldwide.
Nicholas Watt said his flight from London City Airport to Dublin was at the end of the runway when a “smartly dressed man” stood up and began to walk down the aisle, delivering a lecture on climate change.
Cabin crew “calmly and very politely” asked the protester to resume his seat and when he declined they alerted the pilots, Watt said in a tweet.
He said the plane then taxied back to the gate, where police escorted the protester off the plane.
Passengers were then able to reboard the flight, which has since taken off nearly two hours late.
Activists are attempting a three-day “Hong Kong-style occupation of the terminal building” to highlight what they claim is the “incompatibility” of the east London airport’s planned £2bn expansion with meeting the government’s legally-binding commitment to go net carbon neutral by 2050.
Those arriving for flights were redirected to a second terminal entrance by security workers and were not allowed to enter the building without showing their boarding cards first.
Former Metropolitan Police detective John Curran was among those arrested, after he glued himself to the pavement outside the airport.
A spokesman for London City Airport said as of 09:15 BST more than 60 flights which had either arrived or departed.
“We continue to work closely with the Metropolitan Police to ensure the safe operation of the airport, which remains fully open and operational,” he said.
The airport advised passengers to check the status of their flight before travelling.